Vacuum chamber notably for steel decarburisation

ABSTRACT

A vacuum chamber ( 1 ), notably for steel decarburisation, having: an upper part ( 2 ), or the top of the tank, which is in the form of a vacuum bell; and a lower part ( 3 ), or the bottom of the tank, having two jambs ( 4, 5 ) which are fitted with two plungers ( 7, 8 ), namely a rising plunger ( 7 ) and a falling plunger ( 8 ), which are intended to be submerged in the steel in the cavity containing the steel to be treated. The plungers ( 7, 8 ) are connected to the jambs ( 4, 5 ) of the tank by flanges ( 10 ) and a joint ( 11 ) supporting a sealing device. The flanges ( 10 ) are used to mount and disassemble the plungers. The joint ( 11 ) has at least two parallel planes ( 12, 13 ), and forms at least one collar ( 14 ), thereby providing a mortar and tenon-type joint.

[0001] The present invention relates to a vacuum treatment tank, especially for decarburization of steel.

[0002] In order to carry out treatments on steel, output from a converter in which it is melted, it is known to use a vacuum treatment tank usually called an RHOB (Rheinstahl Heraus Oxygen Blowing) tank.

[0003] Such a tank serves to carry out decarburization or dehydrogenization treatments and also light treatments consisting in melting a steel in which the ranges of the various additives are reduced to such a point that it is not possible to obtain them directly in the converter.

[0004] A vacuum treatment tank generally consists of two parts:

[0005] an upper part called the upper tank having the shape of a vacuum bell;

[0006] a lower part, called the lower tank, having two jambs that are provided with two snorkels intended to be immersed in the steel of the ladle containing the steel to be treated.

[0007] During a decarburization treatment of the steel, a quantity of gas, especially argon, is injected by means of injection nozzles located internally and around the periphery of one of the snorkels, called the ascending snorkel, allowing constant circulation of the steel in the lower tank, the return of the steel into the ladle taking place via the other snorkel called the descending snorkel.

[0008] In this type of tank, the snorkels are joined to the jambs of the tank by means of flanges that include a seal and allow said snorkels to be easily mounted and demounted, repaired or changed. The refractory internal parts of the snorkels and of the jamb of the tank are produced by refractory bricks, with a straight seal in the plane of the flange linking jambs and snorkels.

[0009] The straight seal is considered as a very weak part in terms of mechanical and chemical resistance. Erosion, that occurs at this point on the tank, is facilitated by a steel recirculation rate of around 115 metric tons/min for 25 min in the case of low-carbon steels. The circulation is the result of an injected argon gas displacement of around 300 m/s with a flow rate of around 140 Sm³/h.

[0010] This erosion usually results in considerable wear at the jointing face and limits the integrity of the refractory brickwork to around one hundred treatments per campaign, with a stoppage for guniting every four treatments. It is necessary to stop the steel treatment operations for fear a break-out at this point. When the refractory matin face loses effect because of wear, the ingress of air contaminates the steel by nitrogen uptake.

[0011] To avoid this problem, tanks are produced in which the snorkels are fastened to the tank, the flange having been omitted. The metal shell of the snorkels is then welded directly to the tank. This type of tank, that does not allow easy snorkel interchangeability, makes it very difficult to repair the brickwork of the lower tank and results in time wastage and a high cost in using the tank.

[0012] The object of the present invention is to reduce the substantial amount of wear that occurs at the jointing face between jamb and snorkel in a tank having a connection flange between jamb and snorkel.

[0013] The subject of the invention is a vacuum treatment tank, especially for decarburization of steel, comprising:

[0014] an upper part, called the upper tank, having the shape of a vacuum bell;

[0015] a lower part, called the lower tank, comprising two jambs that are provided with two snorkels, namely an ascending snorkel and a descending snorkel that are intended to be immersed in the steel of a ladle containing the steel to be treated, the snorkels being joined to the jambs of the lower tank by means of flanges via a jointing face supporting a seal, the flanges allowing the snorkels to be mounted and demounted. The tank is characterized in that the jointing face comprises at least two parallel planes.

[0016] The other features of the invention are:

[0017] the jointing face forms at least one shoulder, ensuring fitting of the mortise-and-tenon type between the jamb and the snorkel;

[0018] the tenon is produced on the jointing face of the snorkel; and

[0019] the tenon is bricked, fitting into the mortise of the jamb, the mortise also being bricked.

[0020] The description that follows, together with the appended figures, all given by way of non-limiting example, will make the invention clearly understood.

[0021]FIG. 1 shows, in cross section, a tank prior to the invention.

[0022]FIGS. 2A and 2B show, in cross section, a front view and a side view of a tank according to the invention.

[0023]FIG. 3 is a cross-sectional view of the detail of the jointing face according to the invention.

[0024] The invention presented relates, as shown in FIG. 1, to a vacuum treatment tank 1, especially for the decarburization of steel, comprising:

[0025] an upper part, called the upper tank 2, having the shape of a vacuum bell, and intended to create a vacuum above the bath of treated steel, circulating in the tank 1. The tank 1 may include an intermediate ring for protecting the upper tank from wear; and

[0026] a lower part, called the lower tank 3 into which is brought, by lifting, the steel to be treated, the lower part having two jambs 4,5.

[0027] As shown in FIGS. 2A and 2B, the lower tank 3 is provided with two snorkels 7, 8, namely an ascending snorkel 7 and a descending snorkel 8, these being joined to the jambs 4, 5 respectively and designed to be immersed in the steel of the ladle (not shown) containing the steel to be treated. The snorkels 7, 8 are joined to the jambs 4, 5 of the tank 3 by means of a flange 10, on a jointing face 11 supporting a seal. The flanges 10 allow the snorkels 7, 8 to be mounted and demounted. According to the invention, and as shown in detail in FIG. 3, the jointing face 11 comprises at least two parallel planes 12 and 13. The jointing face 11 forms at least one shoulder 14, ensuring a fitting of the mortise-and-tenon type between the jambs 4, 5 and the snorkels 7, 8.

[0028] Preferably, the tenon projects from the jointing face of the snorkel and may include a gently-sloped bevel.

[0029] The tank according to the invention is lined with an internal refractory of the brickwork type and the tenon of the snorkel is also bricked, fitting into the mortise of the jamb, the mortise also being bricked.

[0030] Going into more detail, the snorkel is composed of a metal shell 15 that is bricked up with the refractory bricks of the internal lining. The bricks are bonded together and to the shell by means of a cement 16. On the outside, the metal shell 15 is covered with reinforced concrete 17 produced by casting, in a casting mould that defines the external dimensions of the snorkel, and by inserting, into the volume, the nozzles that are intended for injecting a circulation gas, said nozzles opening around the periphery into the lower part of the snorkel.

[0031] These nozzles inject, for example, argon in order to generate bubbling in the liquid steel, within the space of the snorkel and the space of the jamb in order to modify the density of the steel and generate the movement as depicted in the figures by arrows indicating the circulation of the steel from the ladle into the ascending snorkel 7. The treated steel is then returned to the ladle via the descending snorkel 8, as indicated by the arrow.

[0032] According to the invention, it has been observed that the wear in the region of the plane joint of the tank of the prior art was due to the design of the lower tank, having the jamb joined to the snorkels, of the removable type, on the shell. However, the wear appearing in this region was also due to an effect of the turbulence of the steel owing to the circulation gas injected into the lower part of the snorkel. One solution to the problem posed was to shift part of the plane of the jointing face away from the turbulence region so that the wear behaviour of the jamb/snorkel assembly is reduced in such a way that the tank behaves as a monolithic tank, with in addition the advantage of easy mounting, demounting and changing of the single lower part. It should be noted that the snorkels are subject to considerable wear, especially in their lower end part, which part is in contact with the steel of the ladle. The level of wear is indicated by the hatched part of FIGS. 2A and 2B and shows wear comparable to that occurring in a monolithic tank technique.

[0033] This improvement allows one to anticipate possible integrity over 150 treatments, the wear of the refractory no longer being in principle a point of limitation. The improvement in terms of integrity is 30%. 

1. A vacuum treatment tank (1), especially for decarburization of steel, comprising: an upper part (2), called the upper tank, having the shape of a vacuum bell; a lower part (3), called the lower tank, comprising two jambs (4) (5) that are provided with two snorkels (7) (8), namely an ascending snorkel (7) and a descending snorkel (8) that are intended to be immersed in the steel of the ladle containing the steel to be treated, the snorkels (7) (8) being joined to the jambs (4) (5) of the tank by means of flanges (10) via a jointing face (11) supporting a seal, the flanges (10) allowing the snorkels to be mounted and demounted, characterized in that the jointing face (11) comprises at least two parallel planes (12) (13).
 2. The tank as claimed in claim 1, characterized in that the jointing face (11) forms at least one shoulder (14) ensuring a fitting of the mortise-and-tenon type between the jamb and the snorkel.
 3. The tank as claimed in claim 1 or 2, characterized in that the tenon is produced on the jointing face (11) of the snorkel.
 4. The tank as claimed in one of claims 1 to 3, characterized in that the tenon is bricked, fitting into the mortise of the jamb, said mortise also being bricked. 